Coordinate measuring stages having a friction rod and a motor for each coordinate axis are used in high-accuracy coordinate measuring instruments for measuring substrates in the semiconductor industry. A coordinate measuring instrument having such a coordinate measuring stage is described in the article “Maskenmetrologie mit der LEICA IMS IPRO für die Halbleiterproduktion” [Mask metrology using the LEICA IMS IPRO for semiconductor production] by K.-D. Röth and K. Rinn, Mitteilungen für Wissenschaft und Technik Vol. XI, No. 5, pp. 130-135, October 1997. This measuring instrument is used for high-accuracy measurement of the coordinates of the edges of a pattern element on a substrate, e.g. a mask and a wafer.
The previously unpublished document DE 101 40 174.4-52 describes an additionally improved coordinate measuring stage of the aforesaid kind, and a coordinate measuring instrument for high-accuracy measurement of the coordinates of the edges of a pattern element on a substrate, e.g. a mask and a wafer.
The coordinate measuring instrument has a coordinate measuring stage of the aforesaid kind that is displaceable horizontally in the X direction and the Y direction. It serves to receive a substrate having features whose edge coordinates are to be measured. In addition, a separate interferometer measurement beam path is associated with each coordinate axis (X, Y) of the measuring stage. Measurement mirrors, which are located at the ends of the two interferometer measurement beam paths, are mounted on two mutually perpendicular sides of the measuring stage. By means of the two measurement mirrors, the position of the measuring stage can be determined interferometrically.
The coordinate measuring stage has, for each coordinate axis, a drive unit having a friction bar and a motor. The motor is in contact with its motor shaft against the one end of the friction bar, while an applied pressure roller is in contact against the other end of the friction bar. At least one spring is provided which preloads the applied pressure roller, friction bar, and motor shaft against one another by means of an applied pressure force. The result of this is that the motor shaft engages frictionally against the friction bar, so that the rotational motion of the motor is converted into a linear motion of the friction bar.
Coordinate measuring instruments of the aforementioned kind serve to determine coordinates with a reproducibility in the range of less than 5 nm. Since this measurement accuracy, as already mentioned, depends very substantially on the XY positioning accuracy and vertical running accuracy of the measuring stage, the design of the measuring stage is subject to extremely stringent requirements.
The motors used in the drive units of the known coordinate measuring stages had a rotation-speed-dependent torque. Since the known coordinate measuring stages also already comprised heavy stage components, the motors therefore always needed to be operated at high revolutions in order to generate high torque. In order additionally to achieve good fine-scale positioning in the range below 30 nm with the known coordinate measuring stages, the rotational motion of the rapidly rotating motor shaft therefore had to be greatly reduced using a linkage.
Stepping motors or DC motors, for example, having a friction wheel placed on the motor shaft that acted on the friction bar through a friction wheel reduction linkage, were used for this purpose in the drive units. To ensure sufficient frictional engagement between the last friction wheel and the friction bar, the applied pressure force must be made correspondingly high. The necessary applied pressure force also depends on the mass that needs to be displaced by the drive system.
In the context of a new development of a coordinate measuring machine, a very large XY coordinate measuring stage was required, allowing the mensuration of large substrates in the semiconductor industry, for example 9″ masks and 300 mm wafers. As a result of this, the individual movable stage components (for the X and Y directions) were substantially heavier (up to 80 kg) than in previous coordinate measuring stages. Experiments with this very large XY coordinate measuring stage using the known drive units showed that the known drives did not apply enough torque to move these masses. Because of the large masses to be moved, some of the frictional engagement on the friction bar was lost, i.e. slippage occurred. This resulted, however, in a definite loss of positioning accuracy.